(1) Field of the Invention
This invention relates to a high power semiconductor laser device capable of operating stably in a fundamental transverse mode even at high temperature as a light source for use in an optical communication system, an optical information transmission system and the like, and a method for fabricating the same.
(2) Description of the Prior Art
FIGS. 1(a) and 1(b) are schematic diagrams illustrating, in cross-section, a front view and a side view of a conventional AlGaAs semiconductor laser device, respectively.
In such a semiconductor laser device as shown in FIGS. 1(a) and 1(b), an n-GaAs current blocking layer 22 is grown on a p-GaAs crystal substrate 21 for instance, by an epitaxial crystal growth method, and then a portion of the n-GaAs current blocking layer 22 is subjected to selective etching to form a V-shaped stripe groove 29 having a depth so that a part of the crystal substrate 21 is exposed at the bottom of such groove 29. After the first-step growth, a p-Al.sub.y Ga.sub.1-y As lower cladding layer 23, an Al.sub.x Ga.sub.1-x As active layer 24, an n-Al.sub.y Ga.sub.1-y As upper cladding layer 25 and an n-GaAs contact layer 26 are grown, in the stated order, on the n-GaAs current blocking layer 22 and the groove 29 by the second-step growth where y&gt;x. Thereafter, a pair of n-side and p-side electrodes 27 and 28 are provided to both sides of the semiconductor, respectively. The laser device becomes operative when a forward bias voltage is applied across the n-side and p-side electrodes 27 and 28, and a forward direction current higher than a threshold level is caused to flow in the active layer 24.
With such a semiconductor laser device as described above, an electric current path is limitedly formed in the vicinity of the groove 29 by the formation of the current blocking layer 22 and the groove 29 to thereby concentrate the current in a partial active region of the active layer 24, which corresponds to the width of the groove 29. This structure is an example of inner current confinement structures.
Further, in addition to a double heterojunction in lateral direction, the semiconductor laser device has a loss-guide structure in the transverse direction, where an effective refractive index variation occurs at both sides of the groove 29. More specifically, in the loss-guide structure, the thickness of the lower cladding layer 23 is determined so as to be more thick at a portion corresponding to the width of the groove 29 and so as to be more thin at portions not corresponding to propagation path of a lasing beam, whereby light absorption is accomplished at the current blocking layer 22 whose forbidden band width is smaller than that of the active layer 24, and the width of the groove 29 (approximately corresponding to that of the effective active region) is made narrower to cut-off the higher operation mode. Therefore, it is possible to confine carriers and light in the active layer effectively. To this end, the above described semiconductor laser device is capable of operating with a relatively low threshold current level of 5 mW or less under CW condition at room temperature, having a smooth lasing beam radiation pattern and operating at high temperature in the fundamental transverse mode operation.
While there are almost no problems in the case where the conventional semiconductor laser device is used for producing a relatively low output power, the laser device is disadvantageous in that there may occur a variety of problems in the case where the device is used to produce a relatively high output power of 20 to 30 mW and more. These problems will be described hereinbelow.
As shown in FIG. 1(b), the active layer 24 is entirely uniform in size and in quality to both resonator end facets 30, and therefore the resonator end facets 30 where carriers are poor due to rapid surface recombination, acts as light absorption regions. As the output power of the device increases, the amount of absorption also increases resulting in an occurrence of runaway of a cycle of light absorption, heat generation and an increase in temperature at the resonator end facets 30. When the power exceeds a certain power density level (several mW/Cm.sup.2 in AlGaAs laser device), COD (Catastrophic Optical Damage) may occur resulting in melting damage of the resonator end facets 30. This finally results in such a problem that the semiconductor laser device is out of order.
A variety of laser structures have been proposed in order to improve the output power limit. One example is NAM (Non-Absorbing Mirror) structure in which the forbidden band width of the resonator end facets 30 is made larger than that of the active layer 24 by way of selective diffusion of an impurity using a masking layer of SiN for instance or selective growth of a layer having a wide forbidden band width at the resonator end facets 30, so as to decrease the light absorption at the facets 30. In the NAM structure, the power density is improved nearly one order of magnitude or more by a conventional semiconductor laser device (to several tens of mW/cm.sup.2), as a result of which high output power operation becomes possible. In this case, however, it should be noted that the NAM structure is disadvantageous since it requires an intricate fabricating process requiring a highly accurate control technique. Further, with the NAM structure, in the case of employing no guide means in the transverse direction to the facets 30, extremely large astigmatism may increase remarkably resulting in problems in practical use.
There is another problem in high output power operation that the oscillation mode may become unstable. It is considered that this problem is due to variations in refractive index distribution or current distribution in the active layer which may be caused by high power density or current density. More specifically, the problem resides in the occurrence of a kink (bending) in the light output power versus current characteristics, undesired moving of a lasing output beam or the occurrence of plural peaks in a lasing beam radiation pattern.
In order to improve the above-described unstability in the oscillation mode, a variety of methods have been proposed. Concrete examples thereof are to employ either bury-structure of the active region or an inner current confinement structure, or making the active layer 24 thinner. These methods have been used to make gain distribution (current distribution) and refractive index distribution uniform to thereby stabilize the oscillation mode. However, these methods have been disadvantageous in that extremely high accuracy is required in the control technique, as a result of which the fabrication process becomes highly intricate and thus it is difficult to achieve high reproducability of a semiconductor laser device having preferred characteristics (particularly preferred characteristics in the high output power operation mode).